The molds used in the manufacture of soft (hydrogel) contact lenses have been made from a variety of rigid thermoplastic resins. For example, U.S. Pat. No. 5,540,410 to Lust et al and U.S. Pat. No. 5,674,557 to Widman et al. disclose mold halves made from polystyrene, polyvinyl chloride, polyethylene, polypropylene, copolymers of polystyrene with acrylonitrile and/or butadiene, acrylates such as polymethyl methacrylate, polyacrylontrile, polycarbonate, polyamides such as nylons, polyesters, polyolefins such as polyethylene, polypropylene and copolymers thereof, polyacetal resins, polyacrylethers, polyarylether sulfones, and various fluorinated materials such as fluorinated ethylene propylene copolymers and ethylene fluoroethylene copolymers. Polystyrene is preferred by Widman et al because it does not crystallize and has low shrinkage. An earlier patent, U.S. Pat. No. 4,661,573 to Ratkowski et al, discloses, for the processing of fluorosilicone copolymers into extended wear lenses, molds formed of polypropylene, polyethylene, nylon, Teflon®, glass, or aluminum having its mold surfaces coated with Teflon® polymer.
The manufacturers of soft contact lenses have discovered that if the molds used to make the lenses are sufficiently inexpensive, it is more economical to discard the molds after production of the lenses from the molds than it is to clean the molds to be reused. Polypropylene is a good example of an inexpensive resin that has been used to make molds that can be discarded at minimal cost. Another advantage of polypropylene is that unlike many resins, polypropylene can resist interaction with the monomers used to make the contact lenses. The ability to resist chemical interaction prevents the lens and the mold from adhering to each other and simplifies their separation following lens production.
Despite these benefits, however, polypropylene lens molds also suffer from several known disadvantages. One disadvantage is polypropylene's relatively low dimensional stability. As mentioned by Widman et al, polypropylene partly crystallizes during cooling from the melt and is, therefore, subject to shrinkage, causing difficulties in controlling dimensional changes after injection molding. To improve dimensional stability, manufacturers can make polypropylene lens molds thicker. However, while thicker polypropylene molds can have greater stability, they also require additional cooling time. The additional time needed to cool the thicker molds decreases the number of molds that can be made per machine per unit of time. Furthermore, thicker and therefore larger polypropylene molds can limit the number of molds per machine, thereby reducing product throughput. Finally, polypropylene's relatively poor dimensional stability limits manufacturing yield, because the molds may need to be stored before use, for periods of up to several weeks in some cases, and many polypropylene molds fail to maintain dimensional stability over time to a degree that eventually renders them unfit for lens production.
In addition to having relatively poor dimensional stability, polypropylene has other disadvantages. Polypropylene is a translucent resin that reduces the transmission of light. Typically, polypropylene allows only about ten percent of light to pass through it. Poor light transmission reduces the speed of polymerization. Furthermore, the absorption of oxygen by the molds, commonly experienced with polypropylene molds, can influence lens quality. When the absorbed oxygen diffuses out, during lens molding, polymerization can be affected, and lens surface quality can suffer as a result.
Several alternative resins offer greater dimensional stability and light transmittance than polypropylene. For instance, polycarbonate and polystyrene are more amorphous resins and, therefore, have greater dimensional stability than polypropylene. Moreover, these and other “clear” resins generally transmit at least 50% and often more than 70% of light.
Although polycarbonate and polystyrene resins offer greater dimensional stability and light transmittance, they are vulnerable to chemical interaction with the monomers used in many soft contact lenses (for example, N-vinylpyrrolidone and N,N-dimethylacrylamide, used in many conventional contact lenses). Chemical interaction between the lens monomers and the lens molds can cause the lens and the mold to adhere to each other and, in a worst case scenario, the lens and the mold can become permanently joined. Moreover, in addition to being susceptible to chemical interaction, many clear resins are more expensive than polypropylene and are, therefore, too costly to discard.
Molds for making soft contact lenses have been treated to affect their surface properties. For example, U.S. Pat. No. 4,159,292 discloses the use of silicone wax, steric acid, and mineral oil as additives for plastic mold compositions to improve the release of the contact lens from the plastic molds. U.S. Pat. No. 5,690,865 discloses an internal mold release agent such as waxes, soaps, and oils, including a polyethylene wax having a molecular weight of 5,000 to 200,000 or a silicone polymer having a molecular weight of 2,000 to 100,000. U.S. Pat. No. 5,639,510 to Kindt-Larsen discloses a surface-applied surfactant in the form of a uniform layer or very thin film or coating to assist in the release from each other of mold components of a multi-part mold employed in the molding of hydrophilic contact lenses. Polymeric surfactants that can be used include polyoxyethylene sorbitan mono-oleates which are applied to a non-optical surface of the mold, but do not cover the optical surface of the mold.
U.S. Pat. No. 5,674,557 to Widman et al discloses hydrophilic contact-lens molds that are transiently modified with a removable surfactant to provide a low water dynamic contact angle. This was found to reduce lens hole defects in lens manufacture. Widman et al discloses various polysorbates, ethoxylated amines, or quaternary ammonium compounds that can be applied to the mold surface by swathing, spraying, or dipping.
Mueller et al, in European Patent Application EP 0 362 137 A1, discloses the coating of molds with a co-reactive hydrophilic polymer like polyvinylalcohol, ethoxylated PVA, or hydroxyethyl cellulose, in order to provide a permanent hydrophilic coating on the lens. The mold coating copolymerizes with the lens material in the mold. Similarly, Merill, in U.S. Pat. No. 3,916,033, discloses coating the surface of a mold with polyvinylpyrrolidone to form a coating that is later to come into contact with a previously crosslinked silicone lens. Merill teaches spreading a coating solution over the mold while held in a chuck, thereby achieving a fairly uniform coating of several thousandths of an inch, after which the wet film is allowed to dry to form a hard glassy polymer layer of about 1 to 5 thousandths of an inch. Finally, monomeric N-vinyl pyrrolidone is dissolved in the coating ready for contact with the silicone lens. As one other example, U.S. Pat. No. 5,779,943 to Enns et al. discloses coating a mold with a hydrophobic latent-hydrophilic material, after which a lens material is molded therein. During curing, the mold coating is apparently transferred to the lens surface. The lens is then treated to convert the coating to a hydrophilic form.
It is an object of our system to provide the manufacturer of contact lens and other ophthalmic articles placed on or in the eye with an improved way to mold them, by providing molds with greater light transparency or dimensional stability which can be stored for an extended period of time while, concurrently, maintaining the chemical resistance of the molds to a variety of monomers used in making the ophthalmic articles. This combination of mold properties can be economically achieved by use of the present invention, and may even be achieved in molds that are discarded after a single use.